CalifJim(I'm familiar with the classification (present) subjunctive and the classification mandative construction, but not with the combination subjunctive mandative construction. Is that really "a thing"?)

Indeed, it's a pleonasm. I've been superfluous with words. By the way, is it natural to say: "I've committed a pleonasm"?

CalifJimI think you can safely attribute the acceptance of this form to me personally (and I did say "borderline"). In this case I can't say that I am representative of Americans in general.

I had in mind that in some previous discussions we have said that the present subjunctive (e.g. "it is important that he go", "I insist that he go", etc.) is more common in AmE than BrE, so I wondered whether something similar could be true in this case.

Got it. No, the case of "It's (high / about) time that" shouldn't be grouped with "important" and "insist", etc. for the purposes of understanding American English constructions. My impression is that we're all over the place with what we think should go after "It's time", even though the Ngrams app strongly suggests that it's always "It's time he went", i.e., the past, in published works.